1. The Field Of The Invention
This invention relates to position sensitive input devices. More specifically, the invention is a touchpad surface which responds to a position or a change of position of a probe which is moveable on or above the surface, thereby responding to a change in mutual capacitance between electrode arrays and a shared or common sense electrode caused by interference of the probe.
2. The State Of The Art
Input devices for computers are well known in the art. There are several types of input devices including the familiar "mouse." The mouse has become so popular because, when combined with a graphical user interface (GUI), it is so much easier to use than typed keyboard commands. The mouse has been accepted as a "user friendly" input device for both experienced and novice computer users. The popularity which the mouse has achieved in the art can be given large credit for fostering the explosive growth of the personal computer industry since a mouse provides a simple means for users to input data to a computer.
While mice are currently the most popular non-keyboard input device, a mouse generally requires a free-rolling surface, i.e. a table top, on which it can operate. Disadvantageously, a mouse is not well suited for use in cramped spaces or with a portable computer, particularly laptop, notebook, sub-notebook, and palmtop computers.
In answer to the long existing need for a more convenient input device suitable for both portable and desktop computers, various alternative input devices have been proposed. Such alternative input devices include devices commonly referred to as track balls, track pens, track point devices, as well as various devices which sense the position of a pointing object on a position sensing surface. The devices which sense the position of a pointing object on a sensing surface generally have the advantages of being simple to use, being easily integrated with current computers and other computing devices, reliability, ruggedness, compactness, and the ability to be transported and used in a variety of locations.
Numerous types of input devices utilize a position sensing surface. Examples are provided in the following patent references: U.S. Pat. No. 4,672,154, to Rodgers et al. (Cordless stylus which emits a directional electric field from the tip of a conductive pen cartridge sensed by a digitizer tablet having an X-Y coordinate system); U.S. Pat. No. 4,680,430 to Yoshikawa et al. (A tablet-like coordinate detecting apparatus including a resistive film for determining the coordinate position data of a point on a plane indicated by the touch of a finger tip or other load); U.S. Pat. No. 4,103,252 to Bobick (A position sensing tablet with electrodes located on the boundaries of a sensing region which detects a human touch by the change in capacitance caused by the touch which varies the time constant of an RC network which is part of an oscillator); U.S. Pat. No. 4,736,191 to Matzke (A touch activated control device comprising individual conductive plates wherein a user's touch on the dielectric layer overlaying the plates is detected by individually charging and discharging each of the sectors in the plates in a sequential manner to determine the increased capacitance of the sector); U.S. Pat. No. 4,550,221 to Mabusth (A touch sensitive control device which translates touch location to output signals and which includes a substrate that supports first and second interleaved, closely spaced, non-overlapping conducting plates); U.S. Pat. No. 4,639,720 to Rympalski et al. (An electronic sketch pad which contains a graphics input pad having an array of transparent capacitive pixels, the capacitance characteristics of which are changed in response to the passing of a conductive tipped stylus over the surface of the pad); and, European Patent Publication No. 574,213 (A proximity sensor includes a sensor matrix array which senses changes in capacitance between horizontal and vertical conductors connected to the position sensing pad to determine x, y, & z position information).
A particularly useful and advantageous input device is disclosed in U.S. Pat. No. 5,305,017 issued to Gerpheide (hereinafter referred to as the '017 patent and incorporated herein by reference). In summary, the Gerpheide patent discloses a device and method for overcoming some drawbacks inherent in other input devices which utilize a sensing tablet or sensing surface. The device and method of the '017 patent include a touch sensitive input pad upon which a user conveniently provides input information utilizing a finger or other conductive probe. In typical operation, the user's finger tip is brought in close proximity to a top surface of the position sensing surface of the touch sensitive pad. The touchpad detects the position of the finger tip relative to the X and Y axes in a plane of the touchpad surfaces as well as the finger's proximity in the Z axis in relation to the touchpad surface.
The '017 patent has as one of its distinguishing characteristics the utilization of a measurable decrease in mutual capacitance between electrodes caused by introduction of the finger near the electrodes which are disposed beneath the surface of the touchpad. Mutual capacitance is thereby distinguished from the concept of capacitance of an electrode. The capacitance of an electrode is more commonly defined as the capacitance between the electrode and ground. The concept of utilizing a measurable decrease in mutual capacitance is attributable in part to the use of a virtual dipole electrode (VDE).
The VDE is explained in the '017 patent as being comprised of two index electrode arrays. As shown in FIG. 1, each index electrode array 4 is an array of strip electrodes 6 arranged in a sheet disposed beneath the surface of the touchpad with gaps 8 between the strips 6 to provide electrical isolation. A first array is disposed generally parallel to the surface, and generally perpendicular to an axis along which the position of a probe (i.e. a finger) is to be determined. A second array is disposed in the same plane as the first index electrode array, but rotated at an angle (generally 90 degrees) with respect to the first array. Consequently, the index-index array provides sensing along two axes (the X and Y axes).
The VDE is defined as a virtual dipole because the electrodes can be assigned as belonging to a "P" group, or can be assigned as belonging to an "N" group, where P is defined as a positive edge, and N is defined as a negative edge. Dynamic polarity grouping enables the Ps and Ns to be selectively connected to charge inducing circuitry, and therefore to be switched as desired.
Before discussing how the present invention improves upon the technology disclosed in the '017 patent, it is helpful to further distinguish other state of the art patents from the '017 patent as given above. First, the prior art of Greanias et al., U.S. Pat. No. 4,686,332, differs from the '017 patent because instead of measuring mutual capacitances, Greanias teaches measuring the capacitance of the electrodes with respect to ground. In other words, mutually adjacent vertical and mutually adjacent horizontal conductors having the maximum change in their capacitance are located by finding the electrodes having the greatest increase in capacitance relative to a fixed reference point such as ground. Consequently, while Greanias et al. is concerned with locating conductors having the highest increase in capacitance, '017 teaches measuring the change in mutual capacitance in order to find a linear interpolation region. Ultimately, Greanias et al. fails to achieve the position resolution of the '017 patent because it is limited to resolutions no smaller than the electrode pitch, whereas the '017 patent is able to use linear VDE interpolation to determine resolution much finer than the electrode pitch.
While Greanias fails to accomplish position determination utilizing mutual capacitance measurements, the concept is not unique. For example, U.S. Pat. No. 4,186,392 issued to Holz teaches a keyboard having keypads which are interdigitated with a shared sense electrode. The capacitance between the keypads and the shared sense electrode is measured to determine over which keypad a probe (finger) is positioned, thereby causing a decrease in mutual capacitance. What is important to recognize is that the keyboard is only capable of discrete position determination. In other words, the probe's position can only be determined as being at the discrete locations where a keypad is interdigitated with a sense electrode.
Another patent is issued to Schuyler (U.S. Pat. No. 4,455,452). Schuyler teaches a means for balancing electrode charges for probe position determination. However, the method includes using capacitive coupling to a reference point for position determination, similar to Greanias et al. Furthermore, while Schuyler has much better position resolution than Holz, the position of the probe is not determinable at resolution smaller than the electrode pitch.
Consequently, while the method and apparatus taught in the '017 patent has substantially improved upon the state of the art, position determination still requires that a significant number of measurement calculations be performed. What is needed is a position determining method and apparatus which can take advantage of the high resolution position determination accomplished by the '017 patent, and yet decrease the total number of measurements that must be taken, and consequently realize a reduction in measurement cycle times. It would also be an improvement to reduce the number of wires required to accomplish the measurements, and thereby decrease system complexity.